3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item The application of the attainable region analysis in comminution.(2008-06-09T10:03:28Z) Khumalo, NgangezweABSTRACT This work applies the concepts of the attainable region for process synthesis in comminution. The attainable region analysis has been successfully applied for process synthesis of reactor networks. The Attainable Region is defined as the set of all possible output states for a constrained or unconstrained system of fundamental processes (Horn, 1964). A basic procedure for constructing the attainable region for the fundamental processes of reaction and mixing has been postulated in reaction engineering (Glasser et al., 1987). This procedure has been followed in this work to construct the candidate attainable region for size reduction processes as found in a size reduction environment. A population balance model has been used to characterise the evolution of particle size distributions from a comminution event. Herbst and Fuerstenau (1973) postulated the dependency of grinding on the specific energy. A specific energy dependent population balance model was used for the theoretical simulations and for the fitting of experimental data. A new method of presenting particle size distributions as points in the Euclidian space was postulated in place of the traditional cumulative distribution. This allows successive product particle size distributions to be connected forming a trajectory over which the objective function can be evaluated. The curve connects products from successive batch grinding stages forming a pseudo-continuous process. Breakage, mixing and classification were identified as the fundamental processes of interest for comminution. Agglomeration was not considered in any of the examples. Mathematical models were used to describe each fundamental process, i.e. breakage, mixing and classification, and an The application of the attainable region analysis in comminution Abstract algorithm developed that could calculate the evolution of product particle size distributions. A convex candidate attainable region was found from which process synthesis and optimisation solutions could be drawn in two dimensional Euclidian space. As required from Attainable Region Theory, the interior of the bounded region is filled by trajectories of higher energy requirements or mixing between two boundary optimal points. Experimental validation of the proposed application of the attainable region analysis results in comminution was performed. Mono-sized feed particles were broken in a laboratory ball mill and the products were successfully fitted using a population balance model. It was shown that the breakage process trajectories were convex and they follow first order grinding kinetics at long grind times. The candidate attainable region was determined for an objective function to maximise the mass fraction in the median size class 2. It was proved that the same specific energy input produces identical products. The kinematic and loading conditions are supposed to be chosen as a subsequent event after the required specific energy is identified. Finally the fundamental process of classification was added to the system of breakage and mixing. The attainable regions analysis affords the opportunity to quantify exactly the reduction in energy consumption due to classification in a comminution circuit, thus giving optimal targets. Classification showed the potential to extend the candidate attainable region for a fixed specific energy input. The boundary of the attainable region is interpreted as pieces of equipment and optimum process conditions. This solves both the original process synthesis and successive optimisation problems.Item Household survey on energy consumption patterns in Johannesburg townships: A case study of Diepkloof, Soweto(2007-02-22T10:23:31Z) Mncube, Dumisani WilfredGovernment reforms aim to provide free basic and affordable electricity in poor urban areas. However, the growing rate of urban poverty, limited development of energy infrastructure, the cost of modern services and fuel use culture further contribute towards the consumption of multiple fuel use. The diverse ways in which urban households utilise the available energy can make fuel transition very difficult to achieve. This research aims to investigate household energy consumption patterns in Diepkloof in order to understand the factors limiting a fuel transition from coal and wood to electricity. The analysis in this report uses data collected from the household survey in Diepkloof as well as key stakeholders in this field including the Soweto Electricity Crisis Committee and Eskom. The reason for using these two stakeholders was because SECC is a social movement activist group advocating free electricity while Eskom is the electricity service provider. Household data examines the range of fuels, factors affecting fuel choice and the attitude of people towards alternative energy sources. The two stakeholders help to critically assess the effectiveness of energy policy document supporting the supply of electricity to poor communities, including Diepkloof. This research concluded that there is no clear evidence that an energy transition is taking place in Diepkloof. Moreover, the results indicate that the energy policy has good intentions but there are still some difficulties with affordability, and culture or personal choice. As a consequence of the underlying problem, most poor households still rely on multiple fuels use for major thermal purpose such as cooking and heating.Item Dewatering of Coal Mine Tailings Using Electrokinetics(2006-11-14T08:07:04Z) Sekwele, Matome LudwickIncreasing quantities of finer wastes often contain reactive sulphide minerals and high water contents that pose stability and environmental concerns. This study investigates how electrokinetic process can be improved, to make it more viable towards dewatering finer coal slurries. In the electrokinetic process, a direct current induces the movement of water out of a porous material. A wooden test box was filled up to two-thirds with fine coal slurries. Electrokinetic Geotextiles (EKGs) and brass were used as electrodes. The conducting wires were attached to each electrode and connected to a DC source to form an electro-osmosis cell. Current was passed through the cell and water moved to the cathode where it was withdrawn. The dewatering efficiencies ranged from 13.13 to 109.84 ml/Ah. The energy consumptions ranged from 5.23 to 14.03 kWh/m3 and are in line with those recorded by Johns (2005). Conductivity and pH measurements were taken. EKGs performed better than brass electrodes.